1. Field of the Invention
The present invention relates to a vehicular front monitoring apparatus for recognizing configurations of a road and detecting positions of vehicles and obstacles running and lying ahead of an own-vehicle.
2. Description of the Related Art
As vehicular front monitoring apparatuses for monitoring road situations ahead of an own-vehicle for the purposes of vehicle-to-vehicle distance control and so on, radar-type apparatuses using electromagnetic waves or ultrasonic waves and optical-type apparatuses using CCD cameras, etc. are well known heretofore. A number of techniques relating to such apparatuses are disclosed. In an optical-type front monitoring apparatus, a camera is attached to a front portion of a vehicle, and an image picked up by the camera is processed to extract, e.g., white lines delimiting lanes. Whether a road is straight or curved is determined through computation. Also, when a road is curved, the curvature of the curved road is computed. Accordingly, road situations ahead of an own-vehicle can be precisely detected.
In a radar-type front monitoring apparatus, electromagnetic waves or ultrasonic waves are radiated forward, and based on the waves reflected from an obstacle ahead of an own-vehicle, a motion and position of the obstacle relative to the own-vehicle are detected to determine whether the obstacle exists ahead in the lane where the own-vehicle is running. Regarding road configurations, the curvature of a road is computed based on outputs of a vehicle speed sensor and a yaw rate sensor both mounted on the own-vehicle. The position of the obstacle detected by a radar is corrected depending on the computed curvature of the road so that whether the obstacle exists ahead in the same lane as the own-vehicle is precisely determined. For example, assuming that the yaw rate detected by the yaw rate sensor is .omega. (rad/s) and the vehicle speed is Vs (m/s), a curvature .rho. is given by .rho.=.omega./Vs.
Thus, when the above conventional vehicular front monitoring apparatuses are employed to recognize the curved condition of a road ahead, the following problems are encountered. The optical-type apparatuses can detect the curved condition before reaching a curved portion of the road, but it is necessarily expensive because of the necessity of a camera and an image processing unit. Also, in roads having no white lines to delimit lanes, for example, it is impossible to recognize road configurations. In radar-type apparatuses, since road configurations are judged by detecting the yaw rate as described above, the curvature of a curved road where the own-vehicle is now running can be recognized, but the curvature of a curved portion ahead cannot be detected until the own-vehicle enters the curved portion. Accordingly, a road configuration ahead of the own-vehicle cannot be recognized. For this reason, if a road structure such as a delineator installed on the shoulder of a curved road ahead is detected when an own-vehicle is running on a straight road leading to the curved road, the road structure may be erroneously recognized as a vehicle running ahead, whereupon an alarm is issued or useless slowdown control is performed. In other words, accuracy in front recognition has not been satisfactory.
In vehicular front monitoring apparatuses used in systems for alarming and controlling the vehicle-to-vehicle distance, it is necessary to determine the curvature of a road ahead of an own-vehicle with an inexpensive device. To this end, Japanese Unexamined Patent Publication Nos. 6-68398 and 8-249598 propose techniques to improve the drawbacks of the radar-type apparatuses described above. According to the techniques disclosed in those publications, a road configuration is estimated based on relative positions of delineators installed along a guardrail to an own-vehicle and relative positions of reflectors provided on a plurality of vehicles running ahead, or based on lateral displacements of the relative positions, or based on both the relative positions and the lateral displacements thereof, the delineators and reflectors being detected as parts of obstacles lying ahead by a radar device. With the disclosed techniques, a curved road ahead can be recognized using only a radar system without adding optical means. The disclosed techniques however require complicated computational processing for, e.g., determining whether the detected obstacle lying ahead is a road structure, determining whether the detected obstacle lying ahead is on the left or right side of a curved road, and computing a relative speed.